The demand for Monolithic Microwave Integrated Circuit (MMIC) devices has increased dramatically over the past few years. This increase is due largely to the frequent utilization of MMIC devices in radar systems, electronic warfare devices, missiles and array weapons as well as a wide variety of non-military communications applications. In most cases, there are a number of microwave or millimeter wave components involved, including MMICs, diodes, printed circuits, antennas, and certain waveguide components such waveguide power combiners or waveguide antenna feeds.
These “mixed microwave circuits,” are those in which part of the circuit is in the form of conductively bounded hollow circular or rectangular guides (waveguides), and part of the circuit is in the form of the well known conductor strip sandwiched between parallel dielectric slabs (stripline) or the equally well known conductor strip mounted on a dielectric slab (microstrip). Most of the components utilized for microstrip/stripline transmission lines are typically mounted on planar microstrip transmission line circuits since this method provides manufacturing efficiencies at a relatively low cost.
As the frequency of operation for a given circuit increases, the use of waveguide elements becomes increasingly desirable because of the inherent low loss characteristics associated with waveguide transmission. However, while generally more desirable, waveguide transmission is typically more expensive to implement than microstrip/stripline transmission lines. In addition, since MMICs cannot be mounted directly into a typical waveguide structure, it is generally necessary to transition one or more times between transmission lines of these different types. These commonly implemented transitions between microstrip/stripline and waveguide have also been an issue for certain applications.
However, as the monolithic circuitry in these devices becomes increasingly dense, and as operating frequencies for commercial applications become increasingly popular for broadband applications at K-Band frequencies (18 GHz) through W-band frequencies (94 GHz) and beyond, to include millimeter and sub-millimeter wave ranges, minimizing signal loss becomes an increasingly important consideration. This places a growing burden on existing millimeter wave manufacturing technologies, and especially on radio frequency (RF) input/output transitions, which are often the source of signal capture loss.
The various transition techniques used for channeling high frequency signals in many double-sided or multilayer circuit boards that are connected to a waveguide, typically requires a probe to pass through both the waveguide wall and the circuit board so that when the probe protrudes into the waveguide, it will pick up the signals propagating within the waveguide. In order for such an arrangement to work properly, it is common practice to connect the probe to a microstrip conductor. This is typically accomplished by having the microstrip line on the printed circuit board extend into the side of the waveguide to form an E-plane launch. However, with this arrangement, the transition to the waveguide is often quite “lossy,” and may result in more than 1 dB of loss. Additionally, this arrangement may require hand tuning, using a tuning screw that protrudes into the waveguide, or by other means well know to those skilled in the art. These commonly used practices for assembling and tuning transitions can be quite expensive because of the time and labor associated with assembly and tuning.
Finally, the losses associated with these transitions, which are a combination of both dissipative and impedance mismatch loss, are unacceptable for many applications such as low-noise receivers and certain classes of power amplifiers. Additionally, dissipative and impedance mismatch losses may also result in further degradation or actual loss of signal. Well know methods for tuning, to reduce impedance mismatch losses and improve performance, can increase the cost of devices incorporating these transitions to unacceptable levels for many commercial applications.
In view of the foregoing, it should be appreciated that there is still a need for an efficient, cost effective method and apparatus for coupling microwave or millimeter wave frequency range energy from a microstrip transmission line to a waveguide transmission line. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.